Summary This study assessed associations between ultraprocessed food consumption and dietary nutrient profile linked to obesity in children and adolescents in Argentina, Australia, Brazil, Chile, Colombia, Mexico, the United Kingdom, and the United States using nationally representative data collected between 2004 and 2014. Linear regression models were used to evaluate associations between dietary share of ultraprocessed foods (country and age group‐specific quintiles and a 10% share increase) and the energy density of diets and their content of free sugars and fiber. Ultraprocessed foods, defined by the NOVA system, ranged from 18% of total energy intake among preschool children in Colombia to 68% among adolescents in the United Kingdom. In almost all countries and age groups, increases in the dietary share of ultraprocessed foods were associated with increases in energy density and free sugars and decreases in fiber, suggesting that ultraprocessed food consumption is a potential determinant of obesity in children and adolescents. Effective global policy action to address growing ultraprocessed food consumption and childhood obesity is urgently needed.